A number of ultrasonic cleaning systems have been developed for cleaning irradiated nuclear fuel assemblies including systems utilizing radial omni-directional ultrasonic cleaning technology as described, for example, in U.S. Pat. No. 6,396,892, the contents of which are incorporated herein by reference, in their entirety. FIG. 1 illustrates representative before and after photographs of fuel rods 100 in a fuel bundle cleaned using conventional radial omni-directional ultrasonic cleaning technology. Although, as reflected in FIG. 1, there is clear visual evidence of deposits being removed from the fuel assemblies, the cleaning is neither uniform nor complete, particularly with respect to the peripheral rods.
Comparing cleaning effectiveness data collected from field application of ultrasonic cleaning technology with cleaning effectiveness data collected in laboratory testing indicated that current fuel rod deposits are now exhibiting a dual-layer characteristic comprising both an outer layer that is relatively easy to remove and an inner layer that is much more tenacious. Further, laboratory tests performed by the inventors revealed that the rate of deposit removal achieved with ultrasonic cleaning varies non-linearly with the transducer power applied to the contaminated fuel rod. Accordingly, the deposit removal rate for a given deposit will be relatively low until a threshold ultrasonic power density (PT) is reached, at which point the rate of deposit removal increases dramatically. Similarly, as the tenacity of the deposit increases, the threshold power density required to achieve efficient removal of the deposits increases.
As shown in FIG. 1, there are regions of the fuel where the deposits remained after cleaning with a conventional radial omni-directional ultrasonic cleaning technology. This uneven cleaning has been attributed, at least in part, to non-uniform ultrasonic power density within the cleaning zone. The pattern of clean and dirty regions suggests preferential cleaning in areas that are both aligned with the anti-nodes of the transducers (peak power locations) and exposed to ultrasonic energy from two faces. In these localized higher power density regions, the local power density exceeds the threshold ultrasonic power density (PT) necessary to remove the deposits. It has been estimated that these localized higher power regions may achieve a local power density of approximately twice the bulk power density.
The power density realized at a given location within the cleaning zone depends on several factors, including 1) the total amount of energy output from the transducers, 2) the volume of water into which the ultrasonic energy is transmitted, 3) the degree to which the energy must pass through/around obstructions to get from the transducer to said surface to be cleaned, and 4) any local non-uniformity of the ultrasonic field. The first two factors, together, determine the bulk fluid power density (expressed in watts/gallon (or watts/liter)). Increasing the amount of power or reducing the volume of water results in an increase in the amount of ultrasonic energy (and subsequent cavitation) applied to the cleaning fluid and the surfaces immersed in the cleaning fluid. The third factor (presence or lack of obstructions) affects the distribution of energy within the bulk fluid volume.
As indicated in U.S. Pat. No. 5,467,791, the contents of which are incorporated herein by reference, in their entirety, and from the inventors' laboratory testing, a metallic membrane (such as a fuel channel or cleaning chamber flow guide) may reduce power density by as much as 50% inside the channel/flow guide relative to the power density achieved outside of membrane. The fourth factor (non-uniformity of field) results from localized differences in intensity on the radiating surfaces inherent with both planar and radial omni-directional transducers.
Prior art ultrasonic fuel cleaning systems use various techniques to achieve effective cleaning, including control of cleaning fluid properties, angled orientation of transducers, use of radial omni-directional transducers, and use of reflecting structures to guide energy to the cleaning zone. Although these techniques may provide some cleaning effectiveness benefit, none of the prior art configurations can achieve a power density above the cleaning threshold for the tenacious layer present in current fuel deposits. As shown in Appendix A, the estimated cleaning zone power density of prior art designs is 178 watts/gallon (47 watts/liter) (Kato et al.'s U.S. Pat. No. 5,467,791) and 112 watts/gallon (29.6 watts/liter) ((Frattini et al.'s U.S. Pat. No. 6,396,892) when cleaning a typical pressurized water reactor (PWR) fuel assembly (i.e., 10″×10″ (25.4 cm×25.4 cm) cleaning zone). As will be appreciated, the design disclosed in the Kato patent is specifically tailored for cleaning channeled fuel assemblies (i.e., boiling water reactor (BWR) fuel) and the estimated power density for a PWR version of the Kato design is provided for comparison purposes only.